1. Field of Invention
This invention relates generally to cleaning of heat exchanger tubes. More particularly, this invention relates to the on-line cleaning of a heat exchanger tube and simultaneous improvement of the heat transfer rate across the tube wall.
2. Description of the Related Art
It is well know that the performance of a heat exchanger tube, and therefore the heat exchanger itself, is reduced due to the deposition of material on the inside wall of the tube known as fouling. These deposits reduce the heat transfer rate across the wall of the tube and result in an increased pressure drop across the length of the tube. Further, these deposits can be relatively hard and, therefore, difficult to remove from the tube wall.
Various methods have been developed to clean the inside of a heat exchanger tube to remove these deposits. Most of these methods involve taking the heat exchanger off-line and out of service to manually clean the tubes. These manual methods of cleaning include: high pressure water cleaning to blast away the deposits, acid cleaning to loosen or dissolve the deposits, or the propulsion of a brush or scraping implement through the tube to scrape off the deposits.
All of these manual methods result in the loss of use of the heat exchanger during cleaning and incur the cost associated with the cleaning itself. Furthermore, after cleaning and during operation, the tubes begin to foul and continue fouling resulting in a reduction in heat transfer until the next cleaning. In the case of acid cleaning, pitting and corrosion of the tube my occur.
Other methods for cleaning the tubes without taking the heat exchanger out of service include devices which introduce a number of tube cleaners (e.g., balls or brushes) into the fluid which passes through the tubes. The tube cleaners are designed to fit tightly enough into the tube to contact the tube wall while still being pushed through the tube by the fluid pressure. At the outlet of the tube these tube cleaners are collected and recycled back to the tube inlet. In some systems the tube cleaners are propelled through the tube in a direction opposite the fluid flow by reversing the fluid flow temporarily. The number of tube cleaners used and the recycle rate may vary depending upon the cleaning effectiveness desired.
While these on-line systems avoid having to take the heat exchanger out of service, there is significant cost associated with the necessary piping and valving. Further, these methods are prone to plugging of the tube by debris that has been loosened by the tube cleaners.
After the tubes have been cleaned the pressure drop across the tube and the heat transfer rate across the tube wall return to their nominal design points. However, none of the on-line cleaning methods act to enhance the heat transfer rate across the tube wall.
In view of the foregoing, a need exists for an improved method of cleaning the inside of a heat exchanger tube and for enhancing the heat transfer rate across the tube wall.